Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog is usually expressed as "D-min", the density obtained in the unexposed portions of the emulsion. Density, as normally measured, includes both that produced by fog and that produced as a function of exposure to light. It is known in the art that the appearance of photographic fog can occur during many stages of preparation of the photographic element including silver halide emulsion preparation (which includes nucleation, growth, washing, and concentrating the emulsion), spectral/chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, mixing of the emulsion with coating aids and dye-forming couplers, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging and storage of coated silver halide emulsions.
One form of fog, "reduction fog", originates from the reduction of ionic silver to metallic silver. If this metallic silver forms large enough particles associated with the silver halide crystal the particles are spontaneously developable. Intentional reduction sensitization is also sometimes employed to increase the sensitivity of silver halide grains, but if the particle size of the reduced silver is large enough, there is a similar increase in fog. One means of controlling reduction fog is with materials or conditions that oxidize the large metallic silver centers back to silver ions or to a size too small to spontaneously develop.
Several options are available in the art to facilitate the prevention of reduction fog. Thiosulfonic acids and their salts, as discussed in E.Ger. Patent 7376 (1952); F. Mueller, "The Photographic Image, Formation and Structure"; S. Kikuchi, Ed., Focal, London (1970); and U.S. Pat. No. 5,244,781 have been used during emulsion precipitation and sensitization, and during film formation to oxidize reduction fog. Inorganic oxidants such as mercuric salts, peroxides, persulfates, halogens, sulfur, and permanganates have been described in, for example, EP 0 576 920 A2; and U.S. Pat. Nos. 4,681,838 and 2,728,663 as oxidizing reduction fog, as have organic oxidants such as disulfides, halosuccinimides, or quinones in, for example, U.S. Pat. Nos. 5,219,721 and 4,468,454. All of these examples, and others, have their own limitations. Often fog restrainers have a negative impact on sensitometry, particularly speed. Other may react with dye-forming couplers or may be difficult to use. The use of mercuric salts, which have been universally used as fog restrainers because of their effectiveness, versatility and lack of secondary effects, is no longer desirable due to environmental concerns.
Sulfenimide compounds containing an inhibitor fragment which is released during development have been described in JP 04-194922, JP 04-186346, JP 04-157457, JP 04-350845, JP 63-163337, and DD 251 411. The inhibitor fragment is a silver complexing agent and may inhibit fog formation. Development inhibitors, however, can also impact other film parameters such as interimage effects and contrast. Such effects may be unwanted in the photographic element Further, development inhibitors may have deleterious effects on photographic sensitivity if present before or during latent image formation by either disrupting the kinetics of chemical sensitization, by impeding the formation of latent image, or by poisoning the formed latent image.
Consequently, despite the vast amount of effort which has gone into methods to control fog in photographic elements, there is a continuing need in the industry for practical and environmentally benign stabilizers and fog preventers which do not otherwise adversely affect the performance of the photographic element